You can skip uploading it to your Files if you are just going to use it in your Personal Signature. When you use that editor to create your sig and upload a picture From your Computer it actually stores it in your Files anyway. In a folder there called Signature files.

Wait a few days and see if you can use your old method again. It is a bit shorter, Julea mentioned using Controls V to paste, if you are using a keyboard just click where you want the image to appear but paste it in by pressing Control (Ctrl on many keyboards) and V at the same time. That might work better than use the Paste under Edit has been the last few days.